1. Field of the Invention
The present invention relates to a semiconductor substrate and a process for the same, and more particularly to a semiconductor substrate by bonding and a process for preparing the same.
The present invention is suitably applied particularly to micromachine techniques, dielectric isolation techniques, SOI techniques, sensors, high power devices, communication high frequency band integrated circuit techniques, etc.
2. Related Background Art
Formation of a monocrystalline Si semiconductor layer on an insulator is widely known as a silicon-on-insulator (SOI) technique and has been extensively studied because devices based on utilization of the SOI technique have many advantages that have not been obtained in case of bulk Si substrates for preparing ordinary Si integrated circuits. That is, the following advantages can be obtained by utilizing the SOI technique:
1. Easy dielectric isolation with a possibility of higher level integration, PA1 2. Distinguished resistance to radiation, PA1 3. Reduced floating capacity with a possibility of higher speed, PA1 4. Omission of well formation step, PA1 5. Prevention of latch-up, PA1 6. Possibility to form a fully deplated, field effect transistor by thin film formation, etc. PA1 (1) After the surface oxidation of a silicon monocrystalline substrate, windows are made in a part of the oxide film to partially expose the silicon substrate, and a silicon monocrystalline layer is formed on the SiO.sub.2 by epitaxial growth of silicon in the lateral direction, while utilizing the exposed silicon substrate as a seed. PA1 (2) A silicon monocrystalline substrate itself is utilized as an active layer and an SiO.sub.2 embedded layer is formed below by some means. PA1 (3) After the silicon substrate is bonded to an insulating substrate, the silicon substrate is polished or ground or etched to leave the monocrystalline layer having a desired thickness.
To obtain the above-mentioned many advantages of device characteristics, processes for forming the SOI structure have been studied for these more than ten years. The results are summarized, for example, in the following literature: Special Issue: "Single-crystal silicon on non-single-crystal insulators", edited by G. W. Cullen, Journal of Crystal Growth, Volume 63, No. 3, pp 429-590 (1983).
Among many SOI techniques, SOS (silicon-on-sapphire) formed by heteroepitaxial growth of a silicon film on a monocrystalline sapphire substrate by CVD (chemical vapor phase deposition) method disclosed in the past as a research example has advanced to such a level as to form practical integrated circuits, using monocrystal as a silicon layer and regarded as a successful, matured SOI technique. However, its wide application was interrupted by the occurrence of many crystal defects due to lattice mismatching at the interface between the Si layer and the underlayer sapphire substrate, by diffusion of aluminum into the Si layer from the sapphire substrate, and largely by a high cost of the substrate and delay in response to the formation of larger area.
Recently, some attempts have been made to form an SOI structure without using the sapphire substrate. The attempts can be classified into the following three major groups.
Basically three processes are available as a means of realizing the above group (1): a process for direct epitaxial growth of a monocrystalline silicon layer in the lateral direction by CVD (gas phase process); a process for deposition of amorphous silicon and successive epitaxial growth in the lateral direction in the solid phase by heat treatment (solid phase process); and a process for irradiation of an amorphous or polycrystalline silicon layer with a converged energy beam such as an electron beam, a laser beam, etc. to make a monocrystalline silicon layer grow on SiO.sub.2 through melting and recrystallization or scanning a melt zone by a rod heater bandwise (zone melting recrystallization ) ( liquid phase process ).
These processes have advantages and disadvantages together, and still have many problems in the controllability, productivity, uniformity and quality, and thus have been hardly commercially used yet. For example, the CVD process requires grinding technique of good controllability or sacrificial oxidation to form a flat thin film, whereas the solid phase growth process suffers from poor crystallinity. The beam anneal process has problems in the treating time by scanning with the converged beam, the state of overlapped beams, focus adjustment, etc. Among them, the zone melting recrystallization process is the most matured process, by which relatively large integrated circuits have been made on trial, but there remain still many crystal defects such as quasigrain boundaries, etc. and thus minority carrier devices have not yet been made.
The technique now advanced by research and development by so many research organizations as a means of realizing the above group (2) is a process for forming a SiO.sub.2 layer by oxgen ion implantation into a silicon monocrystalline substrate, called SIMOX (separation by ion implanted oxygen). This technique is now the most matured one because of good matching with the silicon process. However, in order to form the SiO.sub.2 layer, oxygen ions must be implanted at least at 10.sup.18 ions/cm.sup.2 and the implantation time is very long and the productivity is not so high. The wafer cost is also high. Furthermore, there remain many crystal defects. That is, no commercially satisfactory quality for making minority carrier devices can be obtained.
Besides SIMOX a process for forming SOI structure by dielectric isolation on the basis of oxidation of porous silicon is known. The process comprises forming an n-type silicon layer of island shape on the surface of a p-type silicon monocrystalline substrate by proton ion implantation (Imai et al: J. crystal Growth, Vol. 63, 547 (1983)) or by epitaxial growth and patterning, then making only the p-type silicon substrate porous by anodization in a HF solution so as to surround the silicon islands from the surface, and then dielectrically isolating the n-type silicon island by accelerated oxidation. This process has a problem that since the isolated silicon is determined before a device step the degree of freedom in a device design is limited.
As a means of realizing the above group (3), there is a technique of making the mirror surfaces of two wafers where an insulating film is formed on at least one of the mirror surfaces by oxidation, etc. to tightly adhere to each other, subjecting the wafers to a heat treatment to strengthen the bonding at the tightly adhered interface, and then grinding or etching the bonded wafers from one side thereof thereby leaving a silicon monocrystalline thin film having a desired thickness on the insulating film. This technique is called "bonding SOI", and there are many problems to be solved in this technique. The most important problem is in a step of making the silicon substrate into a uniformly thin film. That is, a silicon substrate usually having a thickness of several hundred .mu.m must be ground or etched uniformly to a thickness of several .mu.m or less than 1 .mu.m. This is technically very difficult from the viewpoint of controllability and uniformness. This technique has a possibility to provide a monocrystalline thin film of best quality in the SOI techniques, but has not been used in the commercial production because of the difficulty in the film thickness control.
Another important problem is a generation of a stress due to a difference in the coefficient of thermal expansion between two substrates, where an insulating substrate other than the silicon substrate is used as a support member. That is, there is substantially no problem when a silicon substrate is used as a support member (that is, bonding of two silicon substrates), but when other insulating substrate than silicon, for example, glass, is used as a support member, the substrates are sometimes warped, while keeping the bonding, or cracked or separated from each other due to a difference in the coefficient of thermal expansion in the heat treatment step at about 1,000.degree. C. for strengthening the interface bonding after the two substrates are bonded together. There are cases of synthesizing materials having similar coefficients of thermal expansion to that of silicon and using them as a support substrate, but to our knowledge such synthetic materials have poor heat resistance and cannot endure the heat treatment for strengthening the bonding or the process temperature for forming devices.
Beside these problems, how to suppress the generation of voids (empty spaces formed at the bonding interface) is also a problem. There are many theories on the cause for generation of such voids. According to one theory, oxygen atoms or hydroxy groups covering the surfaces of bonded substrates mainly undergo dehydration and condensation to generate water vapors, and the generated water vapors get together to form voids. The thus generated voids can be made to disappear by further heat treatment at a higher temperature, thereby diffusing the vapors. Surface unevenness caused by dusts, scars etc. on the surface of the substrate will positively generate voids, and the voids generated by the surface unevenness of the substrate are very difficult to make to disappear.
As disclosed, for example, in the first international symposium on semiconductor wafer bonding science, technology, and applications, Extended Abstract of Fall meeting of Electrochemical Society, Phoenis, Ariz. Oct. 13-17, 1991, pp 674-749, the application of substrate bonding technique has been regarded as important in the field of not only SOI, but also micromachine structures, sensors, etc. and a substrate bonding technique capable of suppressing the generation of voids, and releasing the stresses generated at the bonding of substrates having different coefficients of thermal expansion, thereby preventing warpings has been keenly desired.
As described above, the technique capable of providing SOI substrates good enough to prepare electronic devices of improved characteristics with a high productivity has not been established yet.